The Disappearing Beach
A coastal town notices its beach has narrowed over 20 years. Residents blame a new upstream dam built 20 years ago. The town also built a seawall along part of the shore 15 years ago. Waves approach mostly from the northwest. The town has aerial photos from year 1, year 10, and year 20. The beach narrowed most in front of the seawall and downstream (south) of the dam, while a small area north of the seawall gained sand. A student says, 'The dam is the only cause.' Another says, 'The seawall is the only cause.' The class must evaluate both claims using the evidence.
1. Explain how a dam upstream could reduce beach sand, and how a seawall could cause narrowing in front of it. Use the concept of sediment transport.
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A dam traps sediment that would otherwise travel downstream to the coast. With less sediment supply, the beach has less material to replace sand lost to waves and currents, so it narrows over time. A seawall hardens the shoreline: waves reflecting off the wall scour sand from its base, and the wall prevents the beach from migrating inland. Sand is carried away and not replaced, so the beach in front of the wall narrows. Both mechanisms reduce sand, but by different pathways - supply reduction vs. altered erosion.
2. The beach narrowed most in front of the seawall and south of the dam, while a small area north of the seawall gained sand. Evaluate the claim that the dam is the only cause.
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The claim is not supported. If the dam were the only cause, narrowing would be widespread and not concentrated at the seawall. The gain north of the seawall and the strong narrowing in front of the wall point to the seawall as an additional cause (wave reflection and blocked inland migration move sand). The pattern is consistent with both causes acting together, with the seawall locally dominant. The dam explanation alone cannot account for the localized pattern.
3. Propose one measurement or comparison that would help separate the dam's effect from the seawall's effect.
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Acceptable: compare shoreline change at a similar beach with no seawall but downstream of the dam; or compare profiles north and south of the seawall at matched distances from the dam; or measure sediment supply/transport rates upstream and downstream of the dam. The comparison must isolate one variable (presence of seawall or presence of dam) while holding wave exposure and sediment supply as constant as possible.